This invention relates to electronic assemblies and, in particular, to an electronic assembly provided with a composite low flow impedance voltage guard to prevent electrostatic discharge and protect users from hazardous voltages while facilitating the flow of cooling fluid. The voltage guard can be mounted by pick and place techniques.
Open frame electronic assemblies, such as modules or circuit cards, are advantageous over closed frame or covered assemblies in many high power applications. In an open frame assembly, heat generating components such as transformers, resistors and transistors are exposed on the surface of the module or card. Consequently when open frame assemblies are placed in operating environments using moving cooling fluid (e.g. air), the fluid moves in direct contact with the heat generating components. The direct contact permits a high rate of heat transfer from the components to the cooling fluid, enabling higher performance of an open frame assembly as compared with a covered assembly. Thus, for example, an open frame power supply will be able to deliver more power for a given rate of airflow than a closed frame power supply.
Unfortunately, in applications involving assemblies with hazardous voltages, service personnel or users may come into proximity with the assemblies. A hazardous voltage has been defined by pertinent standards (EN60950) as any in-circuit AC voltage over 42.2V (peak) or DC voltage over 60V. In such applications open frame assemblies have the disadvantage that human contact can result in a shock hazard. The shock hazard is typically minimized by disposing the circuitry in a closed frame or covered assembly, but the closed frame or cover blocks or greatly reduces airflow, limiting device and circuit performance.
It is also desirable that the frame or cover should protect nearby voltage-sensitive components, the user, and the ambient environment from electrostatic discharge (ESD). In many applications power components are used in close proximity to low voltage digital circuitry. ESD in such applications can seriously damage such circuitry. Moreover in some hazardous environments ESD can present risk of fire or even explosion.
While there are a variety of frames and covers in use, insofar as is known, none provides ease of cooling, shock prevention and ESD protection in an inexpensive and easily assembled structure. Accordingly there is a need for a new type of voltage guard for an electronic assembly.
In accordance with the invention, an open frame circuit assembly mounted on a planar substrate is provided with a composite low flow impedance voltage guard. The composite guard comprises an ESD protective cover portion and a clip portion to capture the cover portion and clip to the circuit board. The cover portion comprises a lower frame member extending peripherally around the assembly, an apertured top member overlying the assembly and a plurality of spaced apart struts supporting the top member from the frame. The top member and struts have rounded surfaces to preserve streamlines in air flowing over the assembly, and all openings and spacings are sufficiently small to preclude accidental human contact with the assembly. Large area portions of the cover portion are preferably made of polymer containing conductive fillers for ESD protection. The clip portion is preferably made of high elongation polymer for secure holding and clipping.